Switching regulators use negative feedback in their control loops to reduce the effects of parameter variations on output voltage regulation. The presence of feedback in the control loop, however, introduces the possibilities of system instability. Such instability may be initiated by a fluctuation in the load impedance. As with virtually any feedback system, switching regulator control loops must satisfy the Nyquist criterion for insuring closed-loop stability.
The loop gain of a switching regulator (or any feedback system) is a complex quantity that can be represented by its magnitude and phase. The frequency at which the magnitude of the loop gain is one (0dB) is defined as the crossover frequency f.sub.c, and the difference between the phase angle of the loop gain at the crossover frequency and -180 degrees is defined as the phase margin, PM, of the system. The Nyquist criterion states that if the phase margin of the loop gain is less than zero degrees at the crossover frequency, the system will be unstable. How close the phase margin is to zero degrees at the crossover frequency is a measure of the relative stability of the system.
A conventional switching regulator uses an inductor-capacitor (LC) network which filters a pulse-width-modulated (PWM) rectangular switching waveform to produce a relatively constant DC output voltage. The LC filter, however, introduces a large phase shift (commonly called a phase lag) into the control loop of the switching regulator. If this phase lag is not corrected, poor transient response and loop instability can result for the reasons stated above. In most cases, additional compensation circuitry must be added to a switching regulator control loop to compensate for the phase lag introduced by the LC output filter.
For example, in a conventional step-down switching regulator shown in FIG. 1, an LC output filter 10, contributes two poles to the loop gain response. These two poles cause a decrease in the loop gain magnitude and a large phase lag in the forward part of the feedback loop at signal frequencies greater than the resonance frequency of the LC network. Control circuitry within the switching regulator may also contribute high frequency poles. The loop gain magnitude and phase angle plots are shown in FIG. 2.
Because of the gain magnitude decrease and phase lag, the phase margin of the system of FIG. 1 approaches zero or is negative near the unity gain frequency (f.sub.c) of the loop gain. Inadequate or negative phase margin causes oscillatory or unstable closed-loop behavior in a manner analogous to that of improperly compensated operational amplifier circuits. Thus, it is desirable to include circuitry in a switching regulator which compensates for the phase lag of the LC output filter.
Prior art switching regulator designs have utilized phase lead circuits, such as the lead circuit 14 shown in FIG. 3, to compensate the control loop phase response. An illustrative gain magnitude and phase response of the lead circuit 14 is shown in FIG. 4. When this circuit 14 is included in the feedback loop of a switching regulator, it increases the phase margin of the regulator and thus increases its stability.
FIG. 5 shows a phase-compensated step-down switching regulator. It is identical to the switching regulator shown in FIG. 1 except that the phase lead circuit 14 of FIG. 3 has been included in the feedback loop between error amplifier 16 and comparator 18. The loop gain magnitude and phase response plots for the phase-compensated step-down switching regulator are shown in FIG. 6. The phase margin of the compensated regulator is much greater than that of the uncompensated regulator and is therefore much more stable.
Because the capacitor C1 of FIG. 5 must be very large in order to provide phase compensation at the desired frequencies, and large capacitors cannot be economically integrated on an integrated circuit chip, the phase-lead compensation circuit 14 of FIG. 3 has been limited to discrete designs. U.S. Pat. No. 5,382,918 to Mineo Yamatake, assigned to the same assignee as the present application and incorporated herein by reference, describes a capacitance multiplying circuit which may be integrated on a chip; however, that circuit is not suitable for a phase-lead compensation network having characteristics similar to those shown in FIG. 4.
Thus, it is desirable to completely integrate a phase lead compensation circuit within the feedback loop of a monolithic switching regulator. Such a compensation circuit would be required to perform the phase lead compensation function without the use of relatively large capacitors or inductors.